Outwatering takes place permanently from the surrounding rock into such a cavity, and the water collects in the bottom of the cavity. Naturally, such storage can only be used for substances which, in the liquid state, are of lower density than water and which are not miscible with water. The cavity thus contains three superposed phases: at the bottom there is a water phase; above the water there is a liquefied gas phase; and above the liquid surface, the top of the cavity is filled with a gas phase per se. The gas is in equilibrium with the liquid and the pressure is the liquid-vapor equilibrium pressure applicable to the surrounding temperature (generally about 15.degree. C.), with the depth of the cavity being determined as a function of the characteristics of the substance to be stored in order to ensure that it has sufficient hydrostatic pressure.
The storage is used by removing or adding liquid substance, thereby moving the liquid-gas equilibrium surface. The water-liquefied gas phase interface is maintained at a substantially constant level, i.e. it is maintained between two levels which are fairly close together, with water pumps coming into operation whenever the water reaches a given upper level and stopping whenever the water-liquid interface returns to a given lower level.
Such cavity has a bottom which includes a draining well, and generally the bottom has a slight slope towards the well. The upper level for the water is always fixed at some depth below the top of the well, and the liquefied gas extraction orifice is also located within the well, but above the upper water level.
FIG. 1 of the accompanying figures is a diagrammatic cross-section through a common variety of storage facility of the above type. For the purpose of simplifying the explanation, the stored substance is assumed to be butane. However, it must be understood, that the substance could be any other gas which liquefies under pressure, whose liquid density is less than the density of water, and which is not miscible with water in the liquid state.
A cavity 1 has a bottom 3 which slopes gently towards a draining well 2. Four operating ducts are shown diagrammatically: a tube 5 for extracting liquid butane; a tube 6 for pumping out water; a tube 7 for venting gases and opening out into the top of a cavity; and a tube 8 for injecting liquid butane. The water suction orifice 61 is located close to the bottom of the well, and the pump(s) is(are) controlled in such a manner as to maintain the water level between two fixed levels: a top or upper level Ns; and a bottom or lower level Ni. This is quite conventional. The liquid butane suction orifice 51 is placed at a given depth h below the upper of the well, but above the top water level Ns.
When a storage facility is to be "mothballed", i.e. when it is to be taken out of daily use but is not destroyed or damaged, it is conventional practice to fill at least some of the tubes with water or to let the water rise into these tubes up to a level H such that the assembly is in hydrostatic equilibrium. In order to perform this operation safely, the butane suction orifice 51 is placed, for example, at a depth h down the draining well which is sufficient to ensure that the volume of the well above the orifice 51 is greater than the volume of the column of water in the tube(s) up to the equilibrium height H. If the cross-sectional area of the well is noted S and the cross-sectional area of the tube(s) to be filled is noted s, h is determined by the condition: EQU h.S&gt;H.s
This is conventional. Safety is obtained at the cost of increasing the depth of the well, thereby requiring digging to go deeper and requiring the tubes to be longer. In other words safety is obtained at the cost of additional expense relative to placing the butane suction orifice 51 level with the bottom 3. The extra depth can be reduced by increasing the cross-sectional area S of the well. However, this increases the cost of the well. In addition, it is preferable for the water to be pumped from a location where the wall is of limited cross-section, for example in order to have sufficiently accurate information on the rate of outwatering.
Preferred implementations of the present invention reconcile the above contradictory conditions.